1. Field of the Invention
The present invention relates to a projection lens, and more particularly to a projection lens for a video projector to obtain a large picture on a screen by projecting a cathode ray tube (CRT) image.
2. Description of the Prior Art
Projection lenses of a conventional type are comprised entirely of glass lenses or entirely of plastic lenses. U.S. Pat. No. 4,300,817 and Japanese Laid-Open Patent Application No. 57-108818 disclose projection lenses comprising 3-groups consisting of 3 lens elements. U.S. Pat. No. 4,348,081 and Japanese Laid-Open Patent Application No. 58-198017 disclose projection lenses comprising 3 groups consisting of 4 lens elements. If a lens is composed entirely of plastic lens elements, its focal point varies considerably due to changes in atmospheric temperature. Therefore hybrid projection lenses which are comprised of a combination of glass and plastic lens elements have been proposed. A hybrid projection lens comprising 3 groups consisting of 3 lens elements is disclosed in Japanese Laid-Open Patent Application No. 58-125007, and a hybrid projection lens comprising 3 groups consisting of 4 lens element is disclosed in Japanese Laid-Open Patent Application No. 59-155818. The hybrid projection lens is comprised, in sequence from the screen to the CRT, of a positive plastic lens, a positive meniscus glass lens, a positive meniscus plastic lens, and a negative plano-concave plastic lens.
However, in the conventional projection lens, because the first group from the screen has a positive power, the entrance pupil is located at a position inside the lens far from the lens tip at the screen, so that the off-axis light flux is considerably shaded. In other words, the image projected on the screen will lack the proper quantity of light at the periphery of the screen.